1. Field of the Invention
The present invention relates to a reciprocating fluid pump, and more particularly relates to a reciprocating fluid pump and spool valve combination for shifting pneumatic pressure between reciprocating pistons in the pump in order to effect pumping.
2. Description of the Prior Art
Reciprocating pumps are well known in the fluid industry. Such reciprocating fluid pumps are operated by a reciprocating spool valve which shifts pressurized air from one pumping chamber of the pneumatic reciprocating pump to the other as the pumping means (piston, bellows, diaphragm, etc.) reaches the end of its pumping stroke. The valve spool element in the spool valve shifts between two positions which alternately supply pressurized air to the pumping means of one side of the pump while simultaneously permitting the other pumping means to exhaust the air therefrom. The shifting of the valve spool element simply alternates this pressurized air/exhaust between pairs of pumping means within the pneumatic pump, thereby creating the reciprocating pumping action of the pump.
In conventional pneumatic reciprocating pump and spool valve combinations, the valve spool elements have been shifted mechanically or electronically. In mechanical shifting, the spool valve itself is typically constructed as an integral part of the reciprocating pump in a manner such that when the pump piston or diaphragm reaches the end of its pumping stroke, it engages a shift mechanism to mechanically shift the valve spool of the spool valve to its opposite position, which reverses the pressurized air and exhaust to the two reciprocating pumping means in order to reverse the direction of both pumping means to cause the just-exhausted fluid chamber to draw fluid thereinto and simultaneously exhaust (pump) fluid from the opposite full fluid chamber.
In electronic shifting of such a pneumatic reciprocating pump, the mechanical shifting means for the spool valve is replaced with an electric switch or switches which then activate a solenoid operated spool valve for effecting shifting of the valve spool in response to the reciprocating pump pistons', bellows', or diaphragms' having reached the end of their pumping strokes.
A third type of shifting of the spool valve is pneumatic shifting, wherein the pump pistons, bellows, diaphragms, etc. engage mechanical or electrical switches at the end of their respective strokes, which shift the supply air pressure to either end of the valve spool for shifting between positions. In the case of electrical switches, these electrical switches actuate solenoid valves which reciprocate the supply air pressure to the spool valve. A variation of this pneumatically shifted spool valve utilizes pressurized air on both ends of the valve spool, the shifting being effected by the electrical or mechanical switch to release the pressurized air from alternating ends of the valve spool to permit pressurized air at the opposite end to shift the valve spool.
One pneumatically operated reciprocating diaphragm pump on the market today is controlled by a mechanically shifted reciprocating rod that, in turn, causes an internal valve spool within the pump to shift to alternate the applications of pressurized air and exhaust to opposing diaphragm chambers within the pump. The initial shifting mechanism (reciprocating rod) is mechanical, in that it is shifted by being alternately struck on its ends by the two reciprocating fluid pump diaphragms. The alternating rod removes lateral support from a flexible inner sleeve that permits direct pressurized air to bleed around the sleeve to an end surface of the valve spool for shifting the valve spool to its opposite position. Reciprocation of the valve spool reverses the application of pressurized air and exhaust in the reciprocating pump diaphragm chambers in order to effect pumping of the pump, as is customary in all pneumatically operated dual reciprocating diaphragm or bellows-type pumps that are spool valve-actuated.
A similar type of pneumatically actuated reciprocating pump utilizes a spool valve incorporated into the pump body, the spool valve, of course, for reversing pressurized air and exhaust between the two opposed pumping chambers. The pumping chambers comprise connected diaphragms, which diaphragms alternately engage the end of a shifting rod to reciprocate it between left and right positions. The reciprocating shifting rod alternates air pressure and exhaust between the ends of the valve spool to reciprocate the valve spool. Reciprocation of the valve spool, of course, operates the reciprocating pump.
There are many problems associated with the currently available pneumatic reciprocating pumps and spool valve shifting mechanisms. Mechanical shifting of the spool within the spool valve is limited because of available space inside the reciprocating pump, and is also susceptible to premature wear and failure of either the mechanical shifting device for the spool valve, the pump diaphragm or piston itself, or both.
The use of electronics or electrical switching of the spool valve is prohibited in many situations because of the potential for spark and fire hazards generally associated with electric (i.e., spark generating) switching devices, not to mention the complexity that is introduced by the addition of an electric power supply, electrical switches, and solenoid-controlled pneumatic valves.
Some types of pneumatic switching of spool valves in reciprocating fluid pump mechanisms are also a potential source of problems. By providing air pressure to both sides of the spool within the spool valve, the spool has a natural tendency to locate itself in the exact center of the valve when air pressure to the pump is turned off. When it is again attempted to start the pump, the valve spool, being in the exact center of the spool valve, will not direct pneumatic pressure to either side of the valve pumping mechanisms. Therefore, the pump will not be able to start up. This is known in the industry as "deadhead." Deadhead can also occur in mechanical spool valve switches whenever switches on both sides of the pump trip during the same stroke. This can be due to a number of reasons including positive fluid pressure through the pump, the presence of a solid material within the pumped fluid, pneumatic leaks, and of course, mechanical switch malfunction. Air in the pumped fluid within the pumping chamber can also create deadhead problems.
The previously described pneumatically actuated reciprocating diaphragm pump that is actuated by an internal valve spool is difficult to adjust and control, because of the use of the internal deforming sleeve. The valve spool is shifted because the plastic sleeve deforms because it loses its lateral support when the control rod shifts. In theory, when air pressure against the sleeve reaches a predetermined amount, the sleeve will deform, eliminating the air pressure seal between the sleeve and valve spool, causing pressurized air to escape to the end surface of the valve spool to shift it to its opposite position. Because the deformation of the sleeve is so dependent upon a number of external factors (temperature, humidity, presence of lubricants or other chemicals, etc.), it is extremely difficult to predict when and how much the plastic sleeve will deform, and therefore when and how rapidly the valve spool will shift. In addition, constant flexure of the plastic sleeve will create material fatigue brittleness, etc. rendering the sleeve valueless for its intended purpose.
Prior art pneumatically actuated reciprocating fluid pumps have also consistently had problems with pumped fluid surge as pumped fluid from one chamber abruptly stops and fluid from the opposite chamber abruptly starts. This surge causes what is termed hydraulic hammering in supply lines, that tends to vibrate the lines, resulting in unnecessary abrasion, flexure, and fatigue in the lines, and also tends to vibrate the fluid connections and fittings loose near the pump. In certain applications, surge can dislodge particulate contamination within fluid filters and reintroduce this contamination into the fluid system.